Frog sit ups are used to work the abdominals differently than a standard crunch; many people say they are more difficult.
Lie flat on the floor with your legs extended in front of you and your lower back pressed into the mat. Now, bend your knees and place the bottom of your feet together; this will cause your outer thigh to get closer to the floor. Cross your arms in front of your chest and place your hands on the opposite shoulder. Next, pull your torso up like you were going to do a sit up, but stop when you get your shoulders 3-4 inches off the floor. Hold this position and squeeze your abs momentarily. After that, slowly lower your torso back to the starting position. Repeat this movement until you have completed your set.
Pull your legs towards your body, but do not allow your thighs to come up off the floor. Throughout the movement, focus on pushing your thighs down and squeezing your abs.
You can also place your hands on either side of your head, but make sure not to squeeze your neck at all.